1. Field of the Invention
The present invention relates to an optical disk and a method of playing an optical disk. More specifically, the present invention relates to an optical disk which records disk-specific identification information for preventing unauthorized copying, a method of playing the optical disk, and an optical disk drive.
2. Related Art
Conventionally, each optical disk records disk-specific identification information in order to prevent unauthorized copying. As a typical conventional art of recording the identification information on an optical disk, for example, the burst cutting area (BCA) recording method used in a digital versatile disk (DVD) is known.
The BCA recording method is a method configured to form a non-reflective portion on a plurality of tracks by partially fusing a reflective layer of a disk by means of laser trimming, then to dispose this non-reflective portion in a bar code fashion, and thereby to record the identification information on the optical disk which is unique thereto. The BCA recording method is effective for a playback-only disk such as a CD-ROM or a DVD-ROM which only has the reflective layer. However, a BCA writer is very expensive and thereby causes a cost increase.
A write-once optical disk such as a CD-R or a DVD-R does not allow overwriting of the recorded information. Therefore, it is possible to record the disk-specific identification information by use of a recorder. On the contrary, in the case of a magneto-optical disk or a phase-change optical disk, it is possible to overwrite the recorded information. A conceivable method in this case is a method of recording disk-specific identification information by irreversibly changing a recording film using slower line speed and higher laser power than the case of recording normal information (Japanese Unexamined Patent Publication No. Hei9 (1997)-091781), or alternatively, a method of recording disk-specific identification information by forming irreversible marks in a manufacturing step by applying stronger laser power or extending laser radiation time longer than the case of recording normal information, and by alternately disposing regions of the plurality of irreversible marks and regions of normal reversible marks on the outside of a user data area (Japanese Unexamined Patent Publication No. 2002-197670). When the irreversible marks are formed on a rewritable optical disk according to the above-described methods, there is no risk of erasing the formed marks.
However, there is a subtle difference between a signal level of the irreversible mark formed by the above-described method and a signal level of the reversible mark formed by the normal recording power. Accordingly, illegal copy products may be produced by artificially forming illegal marks on a disk, which is not provided with the disk-specific identification information in advance, by use of a normal drive.
Similarly, there is no distinctive difference between the signal levels when the disk-specific identification information is formed in advance as a prepit on a recordable optical disk. For this reason, the disk-specific identification information becomes common at a stamper level. Accordingly, it is necessary to distinguish the optical disks on which the same identification information is formed in advance as the prepit.